1. Field
Embodiments of the present invention relate to image sensors and, in particular, to gate oxides for transfer gates in image sensors.
2. Discussion of Related Art
An image sensor commonly includes several light sensor cells. A typical individual light sensor cell may have a micro-lens, a filter, a photosensitive element, a floating diffusion region, and one or more transistors for reading out a signal from the photosensitive element. One of the transistors is a transfer transistor. The transfer transistor has a transfer gate disposed between the photosensitive element and the floating diffusion. The transfer gate is disposed on a gate oxide. The photosensitive element, floating diffusion region, and gate oxide are disposed on a substrate. The image sensor may be fabricated using complementary metal oxide semiconductor (CMOS) technology or charge coupled device (CCD) technology.
A light sensor cell may operate as follows. Light is incident on the micro-lens, which focuses the light to the photosensitive element through the filter. The photosensitive element detects the light and converts the light into an electrical signal proportional to the intensity of the light detected. The transfer gate transfers the electrical signal from the photosensitive element to the floating diffusion region.
Conventional image sensors work well, but have some limitations. One limitation is that the photosensitive element may not be completely emptied between successive readings. Some of the information from the previous light signal remains in the photosensitive element, having not been transferred to the floating diffusion. The leftover information may be termed image lag, residual image, ghosting, frame to frame retention, etc.
One method of dealing with image lag is to use a dopant underneath the transfer gate. A lateral electric field is created by means of a graded p-type doping of the channel between the photosensitive element and the floating diffusion region, which accelerates the electrons in the channel during readout. However, this can cause two potential problems. One potential problem is a reduction in full well capacity due to the diffusion of the p-type dopant into the photosensitive element. If the p-type dopant is diffused into the photosensitive element, the n-type dopants in the photosensitive element may be compensated and the amount of charge the individual photosensitive element can hold before saturating may be reduced.
A second potential problem is the formation of a potential energy barrier at the region where the photosensitive element connects to the channel underneath the transfer gate. Consequently, not all the photo-generated electrons are able to leave the photosensitive element during readout as some are not energetic enough to cross this potential energy barrier.